1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method, specifically to a high voltage MOS (Metal Oxide Semiconductor) transistor with an increased driving capacity for a level shifter used in an LCD (Liquid Crystal Display) driver, an EL (Electro Luminescence) driver and the like.
2. Description of the Related Art
A semiconductor device according to a conventional art will be explained hereinafter referring to a cross-sectional view of an LOCOS (Local Oxidation of Silicon) offset type high voltage MOS transistor shown in FIG. 9.
A gate electrode 53 is formed on a first gate insulation film 52A and a second gate insulation film 52B on a semiconductor layer 51 of a first conductivity (P-type in this example), as shown in FIG. 9. An N+ source region 54 is formed adjacent one edge of the gate electrode 53. An N− drain region 56 is formed facing to the N+ source region 54 across a channel region 55. And an N+ drain region 57 is formed within the N− drain region 56 and apart from the other edge of the gate electrode 53. A numeral 58 refers to a device isolation film.
The second gate insulation film 52B is thicker than the gate insulating film of a normal voltage (e.g. 10V) MOS transistor. That is to say, for instance, the gate insulation film of the high voltage MOS transistor is about 120 nm thick while that of the normal voltage MOS transistor is about 15 nm thick.
A LOCOS insulation film (the second gate insulation film 52B) is formed on the N− drain region in order to increase a maximum voltage applicable to the device by relieving an electric field concentration in that area.
A weak inversion leakage current increases when the gate insulation film becomes thicker. Therefore raising a threshold voltage by forming an ion-implanted layer in the channel region is necessary in order to suppress the leakage current.
However, on the other hand, when the threshold voltage is raised, a driving capacity of the transistor is reduced.